Differences in the first molt between brood parasitic cowbirds with host-mimetic and host non-mimetic young

FACCHINETTI, C.; URSINO, C.A.; REBOREDA, J.C.

Cusco

Congreso; IX Congreso de Ornitología Neotropical; 2011

Molt and plumage cycles are poorly known in Neotropical birds in general and in South American brood parasitic cowbirds in particular. The Shiny Cowbird (Molothrus bonaeriensis) is a generalist parasite that uses more than 250 hosts while theScreaming Cowbird (M. rufoaxillaris) is an extreme specialist that parasitizes almostexclusively the Baywing (Agelaioides badius). Young of Screaming Cowbirds are mimetic in plumage coloration to young of Baywings while Shiny Cowbird fledglings have female-like plumage. We measured plumage changes of Shiny and Screaming Cowbirds since the time of fledging (i.e. 10 days of age) until young completed their first molt and analyzed species and sexual differences in the molting pattern of body, wings and tail. Screaming Cowbirds started their molt before Shiny Cowbirds (37 vs. 61days of age), but the length of the molt was similar for both species (screaming: 120 days, shiny: 124 days). Screaming Cowbirds always started their molt with under wing covers while Shiny Cowbirds started their molt with crown feathers and in some cases with primaries. There were no sexual differences in molting pattern for both species. Our results show that Screaming Cowbirds start molting in the definitive plumage when they become independent from their foster parents and join conspecifics flocks while Shiny cowbirds start molting one month after joining conspecifics flocks. This shorter latency in the start of molt could facilitate acceptance of Screaming Cowbird young in conspecifics flocks.